Here's a link:
HRM215 Shop Manual on eBay
Now about the slow down problem. Just to be sure, is the engine
and blade slowing down, or just the the blade?
With the blade engaged, the engine should be spinning at 3,150~3,300 rpm. At idle, with the blade stopped, the engine should be at 2,100 /- 150 rpm. If you can get a tach and measure these, we can know for sure what to check next.
There were three Service Bulletins issued for drive-related problems with some of these models. Either the mower would stop self-propel (worn drive pulley), the drive clutch cable might break (improved bend in cable tip), the drive clutch would slip (weak weld in clutch lever). But, none of these would be part of a slow-down of the engine or blade.